Total stations are instruments used in electronic surveying of natural surroundings and structures, particularly for determining distance and angle between the total station and a point of measurement.
In order to aim the telescope of the total station towards a target point of measurement, drive controls are provided for controlling the azimuthal and elevational orientation of the telescope. The total station typically includes an alidade which is rotationally mounted on a base structure, such as a tripod or the like, for rotation about a vertical axis. In addition, the total station comprises a support structure for elevational rotation of the telescope about a horizontal axis.
To track the current azimuthal and elevational orientation of the telescope in the total station, there is provided means for acquiring angular measurement signals in the azimuthal and elevational directions.
The azimuthal angular measurement signal is used to control the drive so as to rotate the alidade to any desired azimuthal orientation as represented by a reference signal.
FIG. 1 shows a simplified elevation view of a total station 100 having a base 105 mounted on a tripod 110, an alidade 115 mounted on the base for rotation about an azimuthal axis 120 and a telescope 125 mounted on the alidade for rotation about an elevation axis 130. The telescope 125 has a sighting axis 135. A controllable drive 140 rotates alidade 115 about axis 120 in response to an azimuth control signal. An angle sensor 145, such as an angle encoder or magnetic or capacitive angle resolver, produces an azimuth measurement signal representing angular position of the alidade relative to the base. Signal processing circuitry including a processor 150 is responsive to the azimuth measurement signal and an azimuthal reference representing a desired azimuthal orientation of alidade 115 for producing the azimuth control signal. The azimuthal reference is provided, for example, by input from an operator using a keypad or knob, or by a tracking subsystem 155 within the total station 100.
A controllable drive 160 rotates telescope 125 about elevation axis 130 in response to an elevation control signal. An angle sensor 165, such as an angle encoder or magnetic or capacitive angle resolver, produces an elevation measurement signal representing elevational angular position of the telescope relative to the alidade. Signal processing circuitry including processor 150 is responsive to the elevation measurement signal and an elevation reference representing a desired elevational orientation of telescope 125 to produce the elevation control signal. The elevation reference is provided, for example, by input from an operator using a keypad or knob, or by a tracking subsystem 155 within total station 100.
In operation, the base 105 is installed on the tripod 110 at a desired angular orientation relative to an azimuthal reference 200 external to the instrument, such as magnetic north, and with azimuthal rotation axis 120 plumb. As the drive 140 rotates the alidade 115, the azimuth measurement signal indicates instantaneous angular orientation of the alidade relative to base 105. At low angular acceleration of the alidade 115, the azimuth measurement signal can also be considered to represent the actual orientation of the alidade 115 relative to the external azimuthal reference 200. In contrast, high angular acceleration of the alidade 115 causes a torsional reaction of the tripod 110 and corresponding angular rotation of the base 105. The torque T1 of the alidade 115 and the opposing torque T2 imposed on the base 105 and on the tripod 110 are shown in FIG. 1. As the angle sensor 145 detects angular position of the alidade 115 relative to the base 105, the azimuth measurement signal does not accurately represent angular orientation of the alidade relative to the external reference 200 when the alidade is subjected to high angular acceleration. Similar torsional reaction skews the elevational angular measurement during high angular acceleration about the elevation axis 130.
Hence, there is a need for instruments and methods which account for such torsional reaction in measuring angular rotation of components subjected to high angular acceleration, particularly for geodetic instruments (such as total stations) incorporating such improvement.